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PLASTER,  O  VERBURNT  GYPSUM 

AND 

HYDRAULIC  GYPSUM 

BY 
M.  GLASENAPP 


An  essay,  describing  the  various  products  obtainable  by  heating  and  calcining  of    native 

gypsum,  based  upon  an  extensive  microscopical  research,  intended  to  throw  light 

upon  the  excellent  properties  of  some  kinds  of  burnt  gypsum  and  to  end 

the  confusion  existing  in  the  classification  of  gypsum  products. 

Illustrated  with  numerous  photographs  of 

microscopic  sections. 


TRANSLATED  BY  DR.  W.  MICHAELIS,  JR. 


(Copyrighted  191 CV,  br  WM.LVM  SEA^F.RT) 


PRICE,  5O  CENTS 


Published  by 

CEMENT  &  ENGINEERING  NEWS 
CHICAGO,  ILL. 


/  >      X 

V 


PLASTER,  OVERBURNT  GYPSUM  AND 
HYDRAULIC  GYPSUM. 

By  M.  Glasenapp. 

An  essay,  describing  the  various  products  obtain- 
able by  heating  and  calcining  of  native  gypsum, 
based  upon  an  extensive  microscopical  research, 
intended  to  throw  light  upon  the  excellent  proper- 
ties of  some  kinds  of  burnt  gypsum  and  to  end 
the  confusion  existing  in  the  classification  of  gyp- 
sum products.  Illustrated  with  numerous  photo- 
graphs of  microscopic  sections. 

Translated  by  Dr.  W.  Michaelis,  Jr. 

(Copyrighted  1910,  by  William  Seafert.) 

The  chemical  and  physical  properties  of  gypsum 
and  of  the  various  commercial  products  obtained 
from  it  by  heating  and  by  calcining  have  so  far  been 
studied  very  little,  so  that  the  most  erroneous  opinions 
on  this  subject  are  daily  expressed  by  men  producing 
and  handling  these  cements. 

Every  one  interested  in  the  manufacture  of  gyp- 
sum and  in  its  wide  application,  therefore,  will  wel- 
come the  news  that  Professor  M.  Glasenapp  has 
made  an  exhaustive  research  of  the  properties  of  the 
various  products  obtainable  by  heating  of  raw  gyp- 
sum at  different  temperatures.  Professor  Glasenapp 
is  in  charge  of  the  technical  laboratory  at  the  Uni- 
versity of  Riga,  Russia,  and  is  considered  an  author- 
ity on  building  materials.  A  thorough  study  of  his 
experiments  will  greatly  benefit  every  person  engaged 
in  the  gypsum  industry,  will  serve  to  spread  knowl- 
edge among  producers  and  consumers  and  to  make 
clear  a  great  number  of  points,  about  which  obscurity 
prevailed. 

The  accurate  work  done  by  this  scientist  will  save 
others,  in  search  of  information,  a  great  deal  of  ex- 
perimenting and  will  be  a  guide  to  them  in  the  prac- 
tical operation  of  their  plants  as  well  as  in  the  mar- 
keting of  their  products  and  in  the  testing  of  the 
manufactured  articles.  Moreover,  the  conclusions 

8 

225262 


which  the  experimenter  draws  from  his  work  con- 
tain a  number  of  valuable  suggestions  how  to  im- 
prove the  commercial  products,  especially  hydraulic 
gypsum  and  Keene's  cement,  in  order  to  make  them 
more  valuable,  and  to  substitute  them  for  other  white 
cements  commanding  a  high  price.  The  large  num- 
ber of  illustrations  accompanying  the  paper  can  not 
fail  to  make  the  subject  lucid  and  interesting  to  the 
reader : 


I.  PLASTER  OF  PARIS  OR  STUCCO  GYPSUM: 

If  ordinary  plaster  of  Paris,  representing  mainly  the 
half-hydrate  CaSO4+^  H2O,  is  mixed  with  water  and 
examined  under  the  microscope,  a  lively  process  ot 
crystallization  can  be  observed  to  set  in  after  5  or  b 
minutes.  In  the  beginning  very  thin  needle-crystals 
form  on  the  cover  glass  and  shortly  afterwards  also 
in  the  liquid  and  on  the  particles  of  gypsum.  After 
15  or  20  minutes  single  needle-crystals  and  groups 
of  them  can  be  seen  in  great  number,  and  especially 
the  larger  fragments  of  the  half-hydrate  are  covered 
with  needle-crystals  radiating  from  them;  also  the 
characteristic  twin  crystals  appear  abundantly.  As 
fast  as  the  crystals  form,  the  original  particles  of 
the  half-hydrate  disappear;  after  an  hour,  they  are 
completely  used  up  and  transformed  into  crystals, 
whereby  the  larger  fragments  of  them  become  the 
centers  of  accumulations  of  crystals,  while  the 
smaller  have  been  converted  into  isolated  star- 
shaped  groups.  Cuts  No.  1  and  No.  2,  illustrate  the 
transformation  of  the  half-hydrate  into  the  crystal- 
lized di-hydrate.  It  may  be  added  that  this  process 
goes  on  in  the  same  way,  no  matter  whether  the  raw 
gypsum  used  in  the  manufacture  of  plaster  was 
crystallized  gypsum  or  gypsum  of  a  fibrous  nature  or 
granular  gypsum. 

After  the  same  gypsum  was-  heated  for  several 
hours  to  a  temperature  of  170  centigrade  (whereby 
the  amount  of  water  still  remained  6.2%,  correspond- 
ing with  the  half-hydrate),  crystallization  began 

4 


after  3  to  4  minutes  and  was  practically  completed 
after  half  an  hour;  only  the  largest  particles  required 
almost  an  hour  to  dissolve  and  to  re-crystallize. 

The  present  state  of  our  knowledge  of  the  harden- 
ing of  gypsum  fe  that,  after  having  been  mixed  with 
water,  the  half-hydrate,  plaster  of  Paris,  and  the  first 
anhydrous  modification  of  gypsum,  v/hich  is  supposed 
to  be  formed  between  130  and  200  centigrade,  form 
over-saturated  solutions,  from  which  the  di-hydrate 


No.     I.     Needle-shaped     crystals     of     di-hyrate        , 
formed  by  mixing  of  plaster  of  Paris  with  water. 
The  plaster  was  produced  by  heating  native  gyp- 
sum to  a  temperature  of  130  centigrade.     Magni- 
fied 360  times. 

precipitates  in  the  shape  of  small  crystals,  a  process 
which  is  finished  only  after  all  half-hydrate  or  an- 
hydrite is  dissolved  and  transformed  into  crystals 
of  di-hydrate.  Apparently  the  half-hydrate  goes  into 
solution  more  rap.dly  and  crystallizes  in  a  shorter 
time  than  the  first  modification  of  anhydrite ;  at  least 
I  conclude  this  from  the  fact  that  this  first  anhydrite, 
which  is  considered  to  be  "easily  soluble,"  dissolves 

5 


the  more  slowly  at  the  higher  a  temperature  it  was 
produced. 

The  same  process  of  solution  and  crystallization, 
disclosed  by  microscopical  examination  of  plaster 
mixed  with  water,  takes  place  also  during  the  setting 
and  hardening  of  plaster  castings,  whereby  the  ratio 
of  plaster  to  water  may  vary  within  wide  limits 
without  materially  influencing  the  time  of  setting.  If 
the  plaster  consists  only  of  the  half-hydrate,  the 
setting  of  the  casting,  according  to  my  observations, 
coincides,  with  the  beginning  of  the  formation  of 


No.  2.  Twin-crystals  and  needle-shaped  crystals 
of  di-hydrate  formed  by  the  going  into  solution 
and  subsequent  crystallization  of  the  particles  of 
half-hydrate  composing  plaster  of  Paris.  Magni- 
fied 360  times. 

crystals  of  di-hydrate,  and  the  final  hardening  corre- 
sponds with  the  complete  transformation  of  the  half- 
hydrate  into  crystallized  di-hydrate.  However,  if  the 
commercial  product  contains  large  amounts  of  the 
first  modification  of  anhydrite — hereafter  referred  to 
as  I.  anhydrite — ,  a  reaction  sets  in  which  accelerates 

«  i ,  I  L,  1 ,  i 


the  hardening;  this  I  shall  describe  later  when  deal- 
ing with  the  so-called  "overburnt  gypsum/' 

Microscopic  sections  of  plaster  castings  readily  show 
the  transformation  which  the  powdered  burnt  gypsum 
undergoes  upon  hardening  with  water,  for,  as  cuts 
No.  3  and  No  4  illustrate,  the  casting  consists  of  a 
dense  network  of  interlacing  needle-crystals  of  Ji- 
hydrate.  Also  crystal-groups  can  be  distinguished 
made  up  of  accumulations  of  twin-crystals  and  nee- 
dle-crystals resembling  cut  No.  2. 

A  burnt  piece  of  raw  gypsum  that  is  to  say  plaster 
of  Paris  in  unground  form,  on  the  other  hand,  ex- 
hibits an  entirely  different  structure  under  the  micro- 
scope. It  shows  parallel  lamination  in  accordance 
with  the  growth  of  the  crystal  (see  cut  No.  5). 

The  fact  that  a  process  of  crystallization  is  the 
cause  of  the  hardening  of  plaster  explains  the  phe- 
nomenon that  water  in  the  form  of  steam  does  not 
harden  it.  Plaster  is  able  to  set  only  in  contact  with 


No.  3.  Microscopic  section  made  from  a  piece 
of  a  hardened  casting  of  plaster  of  Paris.  The 
plaster  used  was  very  quick-setting.  The  crystals 
are  very  small.  Magnified  750  times. 


7 


liquid  water,  in  which  it  can  go  into  solution  and 
crystallize. 

To  judge  from  even  the  most  recent  statements  to 
be  found  in  books  on  chemical  technology,  only  few 
chemists  seem  to  be  aware  of  the  fact,  that  a  com- 
plete transformation  of  every  particle  of  plaster  is 
an  essential  point  in  its  hardening.  Owing  to  the 
greater  solubility  in  water  of  burnt  gypsum  over 
crystallized  gypsum,  the  hardening  of  plaster  of 
Paris  has  been  attributed  to  a  process  of  crystalliza- 
tion for  some  time;  yet,  this  crystallizing  has  mostly 


No.  4.  Section  from  a  casting  of  plaster  of  Paris. 
The  plaster  used  was  less  quick-setting  than  the 
former.  The  crystals  are  larger  than  in  cut  No. 
3.  Magnified  750  times. 


been  regarded  as  of  secondary  importance.  The  pre- 
vailing explanation  was  that  the  partly  or  completely 
dehydrated  gypsum  hydrated,  combined  with  water 
and  hardened,  without  changing  its  form  or  place, 
that  is  to  say  without  previous  going  into  solution. 
The  process,  therefore,  was  considered  to  be  similar 

8 


to  the  hydration  of  calcined  magnesia  and,  in  many 
persons'  opinion,  seemed  to  resemble  the  hardening 
of  Portland  cement  and  of  hydraulic  limes.  This 
erroneous  conception  likewise  led  to  the  belief,  that 
the  strength  of  the  casting  depended  upon  the  hard- 
ness of  the  native  gypsum  from  which  the  plaster 


No.  5.  Microscopic  section  of  a  piece  of  burnt 
natire  gypsum  not  hardened  with  water.  Magni- 
fied 750  times. 

was  burnt.  In  fact,  the  difference  in  hardness  between 
two  kinds  of  raw  gypsum  is  a  matter  of  no  conse- 
quence. The  strength  of  the  hardened  gypsum  de- 
pends solely  upon  the  shape  of  the  crystals,  upon  their 
size  and  upon  their  more  or  less  close  contact.  The 
more  slowly  the  plaster  hardens,  the  larger  and  stronger 
the  crystals  of  di-hydrate  grow,  and  the  less  water  is 
mixed  with  the  plaster  the  denser  and  less  porous  the 
casting  becomes.  Moulds  which  absorb  water  readily, 
therefore,  require  a  plaster  containing  as  little  an- 
hydrite as  possible;  furthermore,  such  moulds  call 
for  a  liberal  amount  of  water.  Admixtures  to  the 
plaster,  which  retard  the  setting,  so-called  negative 

9 


catalysers,  create  large  crystals  and  consequently  are 
the  cause  of  more  resisting  and  stronger  castings. 

The  literature  on  gypsum  contains  contradictory 
statements  about  the  suitability  of  hardened  plaster 
for  the  manufacture  of  plaster  by  re-burning  it.  Most 
authors  say  that  hardened  plaster  is  unfit  for  this 
purpose,  but  admit  they  are  unable  to  give  a  proper 
explanation.  The  answer  to  this  is  that  a  piece  of 
hardened  plaster  consists  of  a  very  porous 
mixture  .of  small  crystals  of  di-hydrate,  which, 
upon  burning  and  subsequent  mixing  with  water, 
behaves  exactly  in  the  same  way  as  when 
mixed  with  water  for  the  first  time  or  as  any  plaster 
burnt  from  raw  gypsum.  This  means  the  fine  inter- 
locking needle-crystals  which,  by  the  re-burning  have 
been  transformed  back  into  the  half-hydrate,  but 
which  are  still  of  the  same  porous  structure,  go  into 
solution  and  crystallize  again  as  di-hydrate,  a  process 
which  may  be  repeated  with  the  same  material  as 
often  as  desired.  The  only  difference  in  the  use  of 
broken  pieces  of  hardened  plaster  instead  of  dense 
raw  gypsum  is  that,  in  the  former  case,  the  raw 
material  for  manufacturing  plaster  is  an  exceedingly 
porous  and  light  mass.  The  consequence  of  this  is, 
that  this  re-burnt  plaster,  as  experiments  will  readily 
show,  requires  more  water  than  the  denser  plaster 
burnt  form  native  gypsum.  Re-burnt  plaster,  there- 
fore, yields  castings  which  are  too  light  and,  for  many 
purposes,  not  sufficiently  resisting. 


10 


II.     OVERBURNT  OR  DEAD-BURNT  GYPSUM, 

SLOW-SETTING  PLASTER, 

DEHYDRATED  GYPSUM: 

The  term  "overburnt  or  dead-burnt  gypsum"  desig- 
nates, in  most  people's  opinion,  a  product  too  slow- 
setting  for  practical  purposes  or  not  hardening  at  all. 
The  literature  does  so  far  not  contain  any  definite  in- 
formation about  the  limits  of  temperature  within  which 
this  kind  of  gypsum  is  obtained.  Rohland  places  the 
limits  between  the  temperatures  of  130  and  525  centi- 
grade. Below  130  centigrade  this  so-called  first  an- 
hydrite (I.  anhydrite)  of  the  calcium  sulphate  is  sup- 
posed to  convert  into  the  half-hydrate  and  above  525 
centigrade  into  hydraulic  gypsum.  The  temperature 
of  130  centigrade  has  been  adopted  by  the  Association 
of  German  Plaster  Manufacturers  as  that  beyond 
which  the  formation,  of  this  anhydrite  begins.  Wheth- 
er or  not  the  half-hydrate  breaks  up  at  once  into 
anhydrite  and  steam,  as  van't  HofT  believes,  or  wheth- 
er a  gradual  process  of  dehydration  sets  in  under  for- 
mation of  various  intermediary  products,  is  so  far  an 
open  question  inconsequental  for  practical  purposes. 

It  is  a  well-known  fact  that  the  anhydrite  burnt  at 
low  temperatures,  say  up  to  240  centigrade,  which,  by 
the  way,  still  contains  0.2  to  0.5%  of  water,  hardens 
rapidly  and  develops  fair  strength,  and,  as  in  the  gyp- 
sum-kettles customary  in  practice  surpassing  of  the 
temperature  of  130  centigrade  can  not  be  avoided,  the 
commercial  plaster  of  Paris  must  necessarily  be  com- 
posed of  a  mixture  of  half-hydrate  and  anhydrite,  or 
more  or  less  dehydrated  half-hydrate.  Gypsum  over- 
burnt  from  a  practical  standpoint  is  produced  only  at 
higher  temperatures,  at  300  centigrade  and  above ;  but, 
even  at  this  point,  temperature  limits  can  not  be  de- 
fined accurately,  as,  in  addition  to  the  temperature, 
the  time  of  heating  is  an  important  factor.  According 

11 


to  Rohland,  the  anhydrite,  obtained  by  prolonged  heat- 
ing of  gypsum  at  temperatures  between  200  and  300 
centigrade,  is  able  to  hydrate,  but  loses  its  capacity  to 
harden.  Such  widely  differing  observations  naturally 
surprise  and  call  for  an  explanation. 

The  microscopical  examination  of  samples  of  powd- 
ered gysum,  burnt  at  temperatures  higher  than  200 
centigrade,  teaches  nothing  essentially  different  from 
the  behavior  of  the  half-hydrate  or  plaster  of  Paris 
towards  water.  Only  the  ability  of  this  I.  anhydrite  to 
form  over-saturated  solutions  is  impaired;  it  is  lim- 
ited the  more  the  higher  the  burning  temperature  has 
been  and  the  longer  the  material  was  heated.  Trans- 
formation into  crystals  of  di-hydrate  takes  place  in  the 
same  manner,  but  more  slowly.  The  following  table 
gives  the  various  temperatures  to  which  the  gypsum 
was  exposed,  as  well  as  the  time  of  heating  in  many 
instances,  and  the  beginning  and  termination  of  the 
process  of  crystallization  corresponding  with  them: 


Temperature.    Burning  Time. 

Beginning  of      Crystallisation. 

Crystallisation     Completed  After 

107  Celsius 

6-7  minutes 

1-2  hours 

130  Celsius 

6-7  minutes 

%  to  i  hour 

170  Celsius 

3-4  minutes 

%  hour 

200  Celsius 

7  hours 

30-35  minutes 

1-2  days 

200-250  Celsius 

14  hours 

60  minutes 

7  days 

250-300  Celsius 

7  hours 

40  minutes 

3  days 

400  Celsius 

%   hour 

I  $4  hour 

17  days 

450  Celsius 

%  hour 

10  days 

30  days 

As  the  hardening  of  the  castings  of  plaster  is  caused 
mainly  by  the  transformation  into  di-hydrate,  and  as 
this  process  of  crystallization  is  the  same  also  for 
"overburnt"  gypsum,  the  lack  of  hardening  in  the  case 
of  the  latter  must  doubtlessly  be  ascribed,  wherever  it 
has  been  observed,  to  the  drying  out  of  the  uncovered 
castings;  the  process  of  crystallization,  therefore,  was 
interrupted  and  the  casting  could  not  obtain  its  full 
strength,  which  it  otherwise  would  have  done.  This 
must  happen  especially  in  cases  in  which  the  process 
of  crystallization  takes  a  number  of  days.  With 
gypsum  burnt  at  200  centigrade  the  transformation 
into  crystallized  di-hydrate  is  almost  completed  within 
24  hours;  only  the  larger  particles  take  more  time, 
and  as,  specially  in  the  case  of  large  castings,  still  a 
sufficient  amount  of  water  remains  for  crystallization 

12 


after  24  or  even  after  48  hours,  this  explains  the  fact 
tha£  gypsum,  burnt  at  200  centigrade  and  even  above 
this  temperature,  unless  heated  for  too  long  a  time,  or 
mixtures  of  this  jvith  standard  plaster  of  Paris,  show 
normal  hardening  and  high  strength.  Rohland  who 
assumes  that  only  a  small  portion  of  the  gypsum,  its 
active  part,  takes  a  share  in  the  hardening,  is  there- 
fore mistaken ;  the  entire  mass  is  active,  if  it  be  given 
time  and  opportunity  to  exhibit  its  activity  which  is 
greatly  diminished  indeed.  Complete  hydration  and 
transformation  into  di-hydrate  without  hardening,  as 
Rohland  describes  it,  is  consequently  out  of  the  ques- 
tion. The  term  "overburnt  or  dead-burnt  gypsum"  is 
therefore  misleading;  the  proper  name  for  gypsum 
burnt  at  temperatures  between  200  and  300  centigrade 
would  be  "slow-setting." 

The  process  of  hardening  of  such  slow-setting  plas- 
ter shows  two  distinct  phases:  In  the  first  place,  the 
plastic  dough  assumes  a  dull  surface  and  becomes 
stiff  owing  to  the  transformation  of  the  anhydrite  into 
the  half-hydrate.  This  point  is  reached  after  1  or 
2  minutes  in  the  case  of  gypsum  heated  to  200  centi- 
grade and  after  30  minutes  or  more  with  gypsum 
burnt  at  temperatures  between  250  and  300  centigrade. 
If  further  absorption  of  water  is  interrupted  at  this 
point  by  a  rapid  drying  process,  the  stiff  plaster  is 
found  to  contain  from  6  to  6.5%  of  water  of  combina- 
tion corresponding  about  with  the  half-hydrate.  Dur- 
ing the  second  phase,  which  requires  more  time,  the 
half-hydrate  previously  formed  goes  into  solution  and 
crystallizes  as  di-hydrate.  Setting  and  hardening  are, 
therefore,  two  well-pronounced  processes  in  this  case. 
Castings  that  have  only  time  to  set  yield  insufficient 
strength;  they  must  be  given  time  to  harden. 

The  more  the  burning  temperature  exceeds  200  cen- 
tigrade and  the  longer  the  gypsum  is  heated,  the  more 
the  anhydrite  loses  its  ability  to  go  back  into  the  state 
of  the  half-hydrate  and  this  latter  in  turn  becomes 
less  capable  to  form  over-saturated  solutions  and  to 
crystallize  as  di-hydrate,  until  finally  a  product  is  ob- 
tained upon  which  water  reacts  so  slowly  that,  for 
practical  purposes,  it  must  be  considered  to  be  over- 
burnt  and  worthless.  But  even  this  kind  of  gypsum 
has  by  no  means  lost  its  capacity  to  harden,  as  Mich- 

13 


aelis  demonstrated  long  ago  by  a  series  of  experiments 
in  which  gypsum  was  heated  from  the  lowest  temper- 
atures up  to  intense  white  heat;  all  of  these  pro- 
ducts hardened  well.  However,  I  must  say  right  here 
that,  at  temperatures  beyond  dark  red  heat,  a  product 
of  entirely  different  properties  is  obtained,  namely 
hydraulic  gypsum  which  so  far  was  considered  to  be 
obtainable  at  much  lower  temperatures. 

The  practice  of  distinguishing  various  kinds  of  an- 
hydrite according  to  their  solubility  as  "easily  solu- 
ble" and"  "insoluble"  or  "difficult  to  dissolve"  may  be 
permissible  for  practical  purposes,  but  is  not  justified 
from  a  scientific  standpoint,  as  the  solubility  decreases 
very  gradually  with  rising  temperatures  and  as  an 
absolutely  insoluble  anhydrite  has  not  been  obtained 
as  yet.  All  that  can  be  said  about  the  behavior  of  the 
I.  anhydrite  towards  water  is  that  its  properties  de- 
pend upon  the  burning  temperature  and  upon  the 
time  of  heating.  First  anhydrite  (I.  anhydrite)  I  call 
all  kinds  of  anhydrous  gypsum  burnt  below  750  centi- 
grade for  reasons  to  be  explained  more  fully  later. 

The  question,  whether  or  not  the  various  grades  of 
slow-setting  plaster,  obtainable  by  the  burning  of 
gypsum  between  200  and  300  centigrade,  can  be  used 
in  practice,  must  not  be  denied,  as  frequently  done, 
not  even,  if  their  hardening  takes  several  days,  a  time 
to  which  we  willingly  accustom  ourselves  in  making 
use  of  Portland  cement.  Such  slow-setting  plasters 
have  certainly  this  advantage  over  ordinary  plaster  of 
Paris  that  the  crystals  of  di-hydrate  grow  the  larger 
the  more  slowly  they  form,  a  circumstance  beneficial 
to  the  grain  and  the  hardness  of  the  casting.  The  rea- 
son, why  castings  made  from  ordinary  quick-setting 
plaster  are  low  in  strength  and  possess  little  resistance 
towards  atmospheric  influences,  is  evidently  to  be 
found  in  the  minute  size  of  the  interlacing  needle- 
crystals  of  di-hydrate  which,  owing  to  the  rapid  pro- 
cess of  crystallization,  have  not  time  to  develop  and  to 
grow  larger.  The  following  table  illustrates  this  point 
by  giving  the  dimensions  of  the  crystals  in  milli- 
meters and  the  corresponding  temperatures  at  which 

14 


the  various   kinds  of  quick-setting  and  slow-setting 

plasters  have  been  burnt: 

Burning  Tfiirperature  Largest  Dimensions  of  Crystals 

Diameter  Length 

107-130  Celsius                                 0.0025  mm.  0.04  mm. 

140     Celsius                                0.012    mm  0.14  mm. 

250-300  Celsius                               0.075    ma^-  -50  mm. 

400     Celsius                                 0.050    mm.  0.35  mm. 

450     Celsius                                o.o3s    mm.  0.60  mm. 

The  plaster  burnt  at  400  centigrade  was  heated  only 
for  half  an  hour ;  that  burnt  at  250-300  centigrade, 
however,  for  several  hours.  This  explains  the  differ- 
ence in  the  dimensions  of  the  crystals.  The  first  two 
of  the  plasters  given  in  the  preceding  table  are  quick- 
setting,  the  last  three  slow-setting. 

The  diameters  of  the  needle-crystals  of  the  plaster 
burnt  at  107-130  centigrade  are  30  times  smaller  than 
those  originating  from  the  plaster  burnt  at  250-300 
centigrade ;  their  sectional  areas  are,  consequently, 
900  times  smaller.  It  is,  therefore,  evident  that,  other 
things  being  equal,  a  casting  made  from  slow-setting 
plaster,  must  show  greater  strength.  Hence,  whenever 
time  has  not  to  be  considered  and  increased  strength 
of  the  casting  is  desired,  as  for  instance  in  the  case 
of  statues  for  art-galleries  and  so  forth,  experiments 
with  slow-setting  plaster  seem  to  be  very  advisable, 
preferably  with  gypsum  burnt  at  a  temperature  be- 
tween 250  and  300  centigrade.  The  moulds  may  be 
removed,  if  necessary,  at  an  early  period,  but  the 
casting  must  not  be  allowed  to  dry  out  before  the 
hardening  process  is  completed.  The  length  of  time 
required  for  the  hardening  can  best  be  ascertained  by 
microscopic  examination.  Samples  taken  from  cast- 
ings made  with  slow-setting  plaster  showed  that  the 
hardening  process  of  the  casting  corresponded  in  time 
exactly  with  the  development  of  crystals  of  di-hydrate 
in  the  microscopic  preparation.  Cut  No.  6,  represents 
a  section  taken  from  a  similar  casting,  magnified  750 
times.  A  comparison  of  it  with  cuts  No.  3  and  No. 
4,  giving  sections  of  quick-setting  plasters  plainly 
shows  the  larger  size  of  the  crystals  of  di-hydrate.  It 
may  be  mentioned,  furthermore,  that  the  slow  process 
of  crystallization  almost  exclusively  gives  birth  to  long 
prismatic  crystals,  while  short  plate  crystals  are  excep- 
tions. 

16 


The  tensile  strength  of  an  "overburnt"  gypsum, 
(slow-setting  plaster)  produced  at  a  temperature  of 
250-300  centigrade^  was  found  to  be,  after  the  comple- 
tion of  the  hardening  process,  270  Ibs.  per  sq.  in.,  a  very 


No.  6.  Section  from  a  casting  made  of  gypsum 
burnt  at  250-300  centigrade  after  having  hardened 
for  5  weeks.  Compare  the  size  of  the  crystals 
with  those  in  cuts  No.  3  and  No.  4.  Magnified  750 
times. 

fair  strength  for  plaster  indeed.  The  surface  of  cast- 
ings made  from  it  has  not  the  chalk-like  appearance  of 
the  common  plaster  of  Paris,  but  possesses  a  glaze  re- 
sembling satin  or  marble ;  moreover  it  is  harder ;  yet, 
it  does  not  obtain  the  strength  of  the  same  plaster 
mixed  with  solutions  of  alum  and  is  likewise  far  in- 
ferior to  hydraulic  gypsum. 

Gypsum  which  may  be  called  "overburnt"  from  a 
practical  standpoint,  that  is  to  say  too  slow-setting  a 
plaster,  is  obtained  between  the  temperatures  of  400 
and  750  centigrade,  yet  even  at  lower  temperatures,  if 
the  heating  is  prolonged  for  several  hours.  The  low- 
er temperature  limit  can,  therefore,  not  be  stated  at 

16 


all,  while  750  centigrade  may  be  regarded  as  the  up- 
per limit,  because  beyond  this  temperature  hydraulic 
gypsum  is  formed. 

Although  worthless  by  itself,  the  plaster  burnt  at 
this  interval  of  temperatures  can  be  made  to  act  very 
energetically,  namely  by  mixing  it,  instead  of  with 
pure  water,  with  solutions  of  certain  salts  which  accel- 
erate the  hardening  process  of  the  common  plaster 
(positive  catalysers).  Thereby  the  "overburnt"  gyp- 
sum regains  its  capacity  to  form  over-saturated  solu- 
tions from  which  the  di-hydrate  crystallizes.  Gyp- 
sum burnt  between  600  and  700  centigrade,  which, 
according  to  Rohland's  statements,  is  unable  to  act 


No.  7.    Transformation  of  gypsum,  "overburnt" 

at  a  temperature  of  600-700  centigrade,  into  crystals 
of  di-hydrate  by  means  of  alum  solution.  To  be 
compared  with  No.  I.  Magnified  360  times. 

even  in  the  presence  of  positive  catalysers,  begins  to 
form  crystals  of  di-hydrate  after  about  45  minutes, 
if  mixed  with  a  solution  of  potassium-alum  and  ob- 
served u»der  the  microscope.  After  from  30  to  48 
hours  it  is  completely  transformed  into  crystals  of  di- 

17 


hydrate.  Cold  saturated  solutions  of  this  salt  yield 
plate-crystals  and  short  prisms  (cut  No.  7  shows  this 
transformation),  while  half-saturated  and  one-quarter 
saturated  solutions  chiefly  give  birth  to  long  prisms. 
The  time  required  for  complete  transformation  into 
the  crystallized  di-hydrate  amounts  to  about  30  hours 
in  the  case  of  saturated  solutions  and  to  approxi- 
mately 48  hours  for  one-quarter  concentration.  Af- 
ter 2  days  the  hardening  process  is  completed  in  every 
instance.  A  gypsum  of  this  kind  possesses  almost 
the  strength  of  hydraulic  gypsum  and  of  Portland  ce- 
ment ;  it  is  so  hard  that  the  thumb-nail  will  not  indent 
it.  The  tensile  strength  was  found  to  be  as  high  as 
525  Ibs.  per  sq.  in. 

To  this  group  likewise  belongs  Keene's  cement  man- 
ufactured by  mixing  of  plaster  of  Paris  with  alum 
solution  and  subsequent  heating  to  red  heat  of  the 
formerly  hardened  mass.  The  powdered  calcined  pro- 
duct is  afterwards  mixed  once  more  with  a  solution 
of  potassium-aluminum  sulphate.  The  same  result, 
however,  is  obtained  by  omitting  the  first  hardening 
with  alum,  that  is  to  say  by  "everburning"  the  raw 
gypsum  in  the  first  place  and  then  gauging  the  powd- 
ered anhydrite  with  alum  solution. 

Another  method  of  making  "overburnt"  gypsum 
active  and  of  accelerating  the  hardening  process  con- 
sists in  admixing  small  amounts  of  quick-setting 
common  plaster.  A  gypsum,  which  was  burnt  at  550 
centigrade  and  mixed  with  water,  was  still  soft  after 
7  days  and  showed  no  sign  of  hardening.  Chemical 
analysis  proved  it  to  contain  only  2.90%  of  water  of 
combination.  The  same  gypsum,  mixed  with  only  W% 
of  its  weight  of  a  plaster  burnt  at  107  centigrade,  con- 
tained 14.88%  of  water  of  combination  after  the  same 
period ;  it  began  to  set  after  several  hours  and  devel- 
oped fair  strength.  This  behavior  must  be  considered 
a  proof  of  Ostwald's  germ  theory  which  attributes 
the  lack  of  hardening  of  "overburnt"  gypsum  to  the 
circumstance  that  it  does  not  contain  undecomposed 
fragments  of  crystallized  gypsum  which  could  act  as 
germs  or  nuclei  and  thus  assist  in  the  process  of 
crystallization. 


18 


III.  HYDRAULIC  GYPSUM: 

During  the  last  years  several  papers  dealing  with 
hydraulic  gypsum  have  been  published.  The  most 
noteworthy  of  them  are  those  by  van't  Hoff  and  by 
Rohland.  Nevertheless,  its  nature,  which  van't  Hoft 
declares  to  be  "quite  mysterious,"  has  not  been  ex- 
plained by  these  contributions.  Scientific  investiga- 
tion and  practical  experience  led  to  a  number  of  con- 
tradictory points,  so  that  this  subject  had  still  to  be 
regarded  as  an  unsolved  problem. 
•  According  to  the  statements  found  in  the  most  re- 
cent literature  on  hydraulic  gypsum  (floor-gypsum, 
flooring-plaster),  this  modification  of  calcined  gypsum 
may  be  briefly  summed  up  to  be  an  anhydrite  of  gyp- 
sum, calcium  sulphate,  which  "under  the  microscope 
in  nitric  acid  shows  the  needle-crystals  of  the  half- 
hydrate,"  which  "has  the  same  composition  as  the 
soluble  anhydrite  and  therefore  must  be  regarded  as 
an  isomorphous  modification  of  it,"  which,  further- 
more, "forms  between  the  temperatures  of  400  and 
600  centigrade"  and  "possesses  the  remarkable  ability 
not  to  assume  the  temperature  of  its  surroundings,  if 
the  latter  surpasses  600  centigrade"  and  which  ulti- 
mately "dissolves  in  the  water  with  which  it  is  mixed 
in  the  same  manner  as  plaster  of  Paris,  though  more 
slowly,  and  crystallizes  as  di-hydrate." 

The  following  description  of  carefully  conducted 
investigations  will  show  how  far  the  real  properties 
of  hydraulic  gypsum  differ  from  the  above  summary 
from  modern  literature: 

Hydraulic  Gypsum  Under  the  Microscope: 

Van't  Kofi's  and  G.  Just's  publication  "The  Hy- 
draulic Gypsum  or  So-Called  Floor-Gypsum"  contains 
a  micro-photograph  of  a  sample  of  gypsum  which  the 

19 


authors  believed  to  represent  hydraulic  gypsum. 
However,  my  microscopical  examinations  of  a  number 
of  samples  of  undoubtedly  genuine  hydraulic  gypsum 
entitle  me  to  make  the  statement  that  the  product, 
which  formed  the  basis  for  the  research-work  of  these 
experimenters,  was  by  no  means  hydraulic  gypsum. 
The  micro-photograph  published  by  them  (see  cut  No. 
8),  as  well  as  the  properties  of  the  gypsum  which  they 
describe  convince  me  that  the  manufacturer  supplied 
them  not  with  hydraulic  gypsum,  but  with  a  sample 
of  "overburnt"  gypsum,  slow-setting  plaster,  so-called 
I.  anhydrite,  which  hardened  fairly  well,  though 
slowly.  It  must  have  been  gypsum  burnt  at  a  temper- 
ature between  250  and  300  centigrade  as  described  in 
the  preceding  chapter.  The  structure  of  the  frag- 
ments of  their  sample  (Cut  No.  8)  indicates  that  a 
fibrous  raw  gypsum  was  used  for  its  manufacture, 
which,  after  having  been  ground,  yields  needle-shaped 
and  prismatic  crystal  fragments. 

That  their  sample  was  merely  slow-setting  plaster 
can,  furthermore,  be  inferred  from  the  remark  by 
the  authors  that  its  capacity  to  harden  was  found  to 
be  decreased  and  the  time  of  setting  retarded  after  it 
had  been  heated  at  a  temperature  of  400  centigrade 
for  10  hours.  From  this  they  rightly  concluded  that 
their  sample  was  burnt  at  a  temperature  below  400 
centigrade.  A  similar  behavior,  however,  can  only 
be  discovered  in  the  case  of  the  above  described  slow- 
setting  gypsum,  but  is  not  characteristic  of  hydraulic 
gypsum,  which  is  obtained  only  at  temperatures  far 
above  400  centigrade,  as  will  be  shown  later,  and 
upon  which  heating  at  400  centigrade  has  no  influ- 
ence whatever. 

This  confounding  of  hydraulic  gypsum  and  slow- 
setting  plaster  is  easily  explained  by  the  fact  that  in 
commerce  these  two  kinds  of  gypsum  are  frequently 
not  properly  distinguished  one  from  the  other  and 
that  the  hardening  process  of  both  of  them  shows  this 
resemblance  that  it  is  completed  only  after  weeks 
and  months,  which  plainly  distinguishes  them  from 
common  plaster,  Unfortunately,  however,  yan't 
HofFs  publication,  describing  a  gypsum  with  entirely 
different  properties,  has  been  widely  quoted  and  thus 
has  spread  a  wrong  conception  of  the  method  of  pro- 

20 


duction  and  of  the  properties  of  hydraulic  gypsum. 
Also  Rohland  has  been  influenced  by  it;  as  a  conse- 
quence, the  latter's  publication  does  not  contribute 
anything  towards  explaining  the  actual  properties  of 
hydraulic  gypsum;  on  the  contrary  it  makes  the  con- 
fusion even  greater. 


No.  8.  Slow-setting  plaster.  Micro-photograph 
taken  from  van't  Hoff's  and  Just's  publication.  By 
them  believed  to  be  hydraulic  gypsum  or  floor-gyp- 
sum. 


Remarks  by  the  Translator: 

The  American  reader  can  best  obtain  an  idea  of  the 
confusion  existing  in  the  classification  of  gypsum  pro- 
ducts and  of  the  unreliableness  of  statements  to  be 
found  in  modern  literature  by  referring  to  the  fourth 
chapter  of  Edwin  C,  Eckel's  book  on  "Cements,  Limes 
and  Plasters,"  which  contains  a  translation  of  the 
above  mentioned  publication  by  van't  Hoff  and  Just. 

Also  Eckel  quotes  van't  Hoff  with  the  greatest 
reverence.  He  divides  the  gypsum  products  obtain- 
able at  higher  temperatures  than  customary  for  the 

21 


burning  of  plaster  of  Paris  into  two  groups,  "Floor- 
ing-Plasters" and  "Hard-Finish  Plasters,"  and  gives 
the  first  name  to  all  grades  of  gypsum  "prepared  by 
simple  burning  at  high  temperatures"  and  the  second 
to  those  "produced  by  a  double-burning  with  the 
additional  use  of  chemicals." 

He  continues :  "Neither,  product  is  made  to  any  ex- 
tent in  the  United  States,  though  a  considerable 
quantity  of  hard-finish  plasters  are  imported  every 
year.  The  data  obtainable  as  to  processes  of  manu- 
facture arje  scanty,  and  the  descriptions  published  are 
often  contradictory,  so  that  it  has  been  difficult  to 
prepare  a  satisfactory  account  of  these  products..  It 
is  believed,  however,  that  the  descriptions  given  below 
contain  no  errors  of  importance." 

"The  flooring-plasters  ("Estrichgips"  of  German 
reports)  include  those  plasters  made  by  calcination  of 
a  relatively  pure  gypsum  at  temperatures  of  400 
Fahrenheit  (equal  to  200  centigrade)  or  higher." 
""In  the  literature  of  gypsum  and  plaster  it  is  often 
stated  that  gypsum,  burned  at  temperatures  exceed- 
ing 400  Fahrenheit,  yields  a  completely  dehydrated 
product — an  artificial  anhydrite — which  is  entirely 
valueless  as  a  structural  material,  because  it  has  com- 
pletely lost  its  property  of  recombining  with  water. 
This  statement  is,  however,  erroneous,  for  plasters 
burned  at  such  temperatures  are  regularly  made  and 
used.  They  set  with  extreme  slowness,  however,  and1 
require  very  fine  grinding." 

"Until  very  recently  no  satisfactory  discussion  of 
this  phenomenon  had  been  attempted,  and  the  few 
published  accounts  of  the  manufacturing  processes 
employed  were  contradictory  as  to  temperature 
reached,  composition  of  product,  etc.  Fortunately, 
however,  a  detailed  account  of  the  chemical  changes 
involved  was  published  during  1903  by  van't  Hoff  in 
the  Transactions  of  the  Berlin  Academy  of  Sciences. 
As  this  paper  is  practically  inaccessible  to  the  Amer- 
ican engineer  or  manufacturer,  a  translation  is  here 
appended:" 

Then  follows  van't  HofFs  and  Just's  publication  on 
"The  Hydraulic  Gypsum  or  So-Called  Floor-Gyp- 
sum," which,  as  demonstrated  by  Glasenapp,  refers 
to  a  gypsum  burnt  below  400  centigrade,  to  a  slow- 


setting  plaster,  whereas  hydraulic  gypsum,  or  the 
floor-gypsum  of  commerce,  is  obtained  only  at  tem- 
peratures above  750  centigrade.  Van't  Hoff  con- 
cludes his  paper  as  follows: 

"The  essential  result  of  the  investigation,  there- 
fore is,  that  in  the  heating  of  gypsum  after  total  de- 
hydration, which  occurs  at  about  190  centigrade,  the 
capacity  to  bind  water  is  at  first  retained,  and  is  only 
gradually  lost,  either  by  more  intense  or  by  longer 
heating.  The  retention  of  the  crystalline  form,  which 
is  probably  due  to  burning  without  previous  division 
into  small  bits,  checks  this  so-called  dead-burning, 
and  is  therefore  of  technical  importance.  We  found 
no  evidence  to  support  the  statement  that,  after  dead 
burning,  a  new  binding  capacity  appears  at  a  high 
temperature,  in  which  case  even  the  natural  anhydrite 
would  be  suitable  for  burning  floor-gypsum." 

In  the  face  of  this  meagre  information  available  and 
involuntary  misrepresentation  on  the  part  of  van't 
Hoff,  the  reader  of  Glasenapp's  present  publication 
must  necessarily  admire  the  thoroughness  of  his  re- 
search-work and  be  thankful  to  the  experimenter  for 
the  large  amount  of  data  and  information  supplied. 
This  paper,  in  my  opinion,  will  elucidate  the  confu- 
sion that  existed  heretofore  and  will  give  rise  to  a 
number  of  additional  essays  until  finally  manufact- 
urer, consumer  and  inspecting  chemist  will  be  equally 
educated  on  the  subject  and  will  all  be  convinced  of 
the  merits  of  the  higher  grades  of  gypsum,  namely 
slow-setting  plaster  and  hydraulic  gypsum. 

We  now  return  to  Professor  Glasenapp's  paper: 

The  literature  on  hydraulic  gypsum  contains  fre- 
quent references  to  "crystals  of  the  half-hydrate." 
To  this  I  must  remark  that  the  half-hydrate  consti- 
tuting- plaster  of  Paris  does  not  possess  a  crystal 
shape  of  its  own ;  what  we  see  under  the  microscope 
are  the  crystal  fragments  of  the  partly  dehydrated 
raw  gypsum.  These  fragments,  contained  in  plaster 
as  well  as  those  found  in  "overburnt"  gypsum  up  to  a 
burning  temperature  of  800  centigrade,  have  the  shape 
of  splinters  and  spikes  or  resemble  small  rods  and 
pins,  if  the  raw  gypsum  had  a  fibrous  structure.  On 
the  other  hand,  genuine  hydraulic  gypsum  shows,  even 
if  magnified  only  300  or  500  times,  a  peculiar  gran- 

23 


ular  structure.  After  washing  out  of  the  fine  parti- 
cles, the  coarse  feels  as  hard  as  sand  grains  and  is 
difficult  to  grind.  These  sintered  particles  possess 
polygonal  shape  and  have  sharp  or  rounded  corners; 
they  are  double-refracting.  The  laminated  structure 
of  the  crystal  fragments  of  raw  gypsum,  however, 
which  becomes  visible  only  upon  driving  off  of  the 
water  of  crystallization  at  low  temperatures,  and 
which,  therefore,  characterizes  plaster  of  Paris  and 
slow-setting  plaster,  is  not  to  be  found  in  the  case  of 
hydraulic^  gypsum.  Cut  No.  9,  shows  the  coarser 
particles  of  genuine  hydraulic  gypsum,  commercial 
product;  cut  No.  10,  is  the  same  preparation  as  No. 
9,  but  in  this  case  the  coarse  particles  have  been 
broken  up  by  grinding  them  between  cover  glass  and 


No.  9.  Powdered  hydraulic  gypsum  (commer- 
cial product)  after  washing  out  of  the  finest  par- 
ticles. The  coarse  consists  of  a  multitude  of 
grains  fused  together.  They  can  be  separated  by 
pressure  on  the  cover  glass  and  then  present  the 
view  given  by  cut  No.  10.  Magnified  200  times. 


24 


slide.  Cut  No.  11,  represents  powdered  plaster  of 
Paris,  commercial  product,  for  comparison.  This  last 
cut  is  likewise  an  illustration  for  slow-setting  plaster, 
"overburnt"  gypsum. 


No.    10.    Grains    composing    hydraulic    gypsum. 
Same  preparation  as  No.  9.     Magnified  360  times. 


The  shape  and  optic  properties  of  hydraulic  gyp- 
sum are  so  characteristic,  that  it  is  impossible  to 
mistake  it  for  plaster  of  Paris.  Even  a  small  admix- 
ture of  one  kind  to  the  other  can  easily  be  detected 
by  the  microscope.  A  further  method  of  distinguish- 
ing them  furnishes  the  examination  under  crossed 
Nichols,  whereby  hydraulic  gypsum  shows  brilliant 
colors.  The  grains  and  other  elements  of  which  the 
hydraulic  gypsum  may  be  composed  are  crystalline, 
double-refracting  and  apparently  belong  to  the  rhom- 
bic system.  If  placed  in  water,  they  do  not  change 
their  form,  not  even  after  several  months,  and  are 
not  inclined  to  re-crystallize  as  di-hydrate. 

25 


What  Temperature  is  Required  for  Burning  Hy- 
draulic Gypsum?  In  looking  for  an  answer  to  this 
important  question  in  the  available  literature,  one 
finds  nothing  but  uncertainties  and  conjectures. 
While  the  temperature  interval  of  525  to  600  centi- 
grade was  formerly  declared  to  be  the  proper  burn- 
ing temperature,  modern  investigators  place  it  as 
low  as  400  and  even  between  200  and  400  centigrade. 
But  that  in  all  the  numerous  observations  on  record 
"overburnt"  gypsum,  slow-setting  plaster,  was  used 
instead  of  hydraulic  gypsum  is  evident  from  the 
description  of  the  properties  of  the  materials  under 
examination  and  from  the  accompanying  micro-photo- 
graphs. All  these  various  grades  of  gypsum  had 
nothing  in  common  with  the' commercial  genuine  hy- 
draulic gypsum  which,  as  every  manufacturer  of  this 
product  knows,  can  only  be  obtained  at  a  bright  red 
heat  corresponding  with  a  temperature  of  from  900 
to  1000  centigrade. 


No.  IT.  Powdered  plaster  of  Paris  (commercial 
product).  All  kinds  of  gyp?um  obtainable  by  burn- 
ing between  107  aid  800  centigrade  show  these 
same  elements.  Magnified  360  times. 

26 


In  order  to  ascertain  the  limits  of  temperature  be- 
tween which  the  granular  modification  of  the  an- 
hydrite is  formed,  which  microscopic  investigation 
proved  to  be  hydraulic  gypsum,  small  samples  of 
various  kinds  of  raw  gypsum  were  burnt  in  a  furnace, 


No.  12.  Hydraulic  gypsum  burnt  at  900  centi- 
grade. Section  through  an  unbroken  piece  showing 
parallel  stripes  originating  from  the  fibers  of  the 
raw  gypsum.  The  dark  spots  are  adulterations  of 
clay.  Magnified  750  times. 


heated  by  electricity,  at  temperatures  of  600  centi- 
grade, 700,  800  and  so  on,  the  temperature  of  each 
following  burning  being  100  centigrade  higher.  The 
heat  was  measured  by  a  Le  Chatelier  pyrometer.  The 
burning  time  was  from  3  to  4  hours  which  was  suffi- 
cient for  pieces  of  gypsum  of  nut  size.  Microscopic 
examination  of  the  products  obtained  had  the  fol- 
lowing result: 

The  structure  of  the  samples  burnt  at  600  and  700 
centigrade  could  not  be  distinguished  from  that  of 
plaster  of  Paris.  These  temperatures,  therefore,  yield 

27 


exclusively  "overburnt"  gypsum.  The  product  burnt 
at  800  centigrade,  however,  if  slightly  magnified, 
showed  still  the  same  laminated  structure  character- 
istic of  dehydrated  gypsum  for  all  temperatures  up  to 
700  centigrade  and  as  shown  by  cut  No.  5.  Upon 
more  powerful  magnification,  the  laminae  could  be 
discerned  to  consist  of  minute  grains,  of  the  shape 
of  those  to  be  found  in  hydraulic  gypsum,  especially 
at  the  thin  edge  of  the  specimen  under  observation. 


No.  13.  Hydraulic  gypsum  burnt  at  900  centi- 
grade. Section  through  an  unbroken  piece  showing 
the  granular  elements  composing  it  in  their  original 
place.  Magnified  750  times. 


Mixed  with  water,  this  sample  hardened  well  and 
acted  like  hydraulic  gypsum.  The  granular  modifica- 
tion of  anhydrite,  however,  attains  its  full  develop- 
ment only  at  a  temperature  of  900  centigrade,  when 
microscopic  sections  present  the  structure  given  by 
cuts  No.  12  and  No.  13. 


Cut  No.  12  shows  the  less  frequent  occurrence  of 
grains  arranged  in  parallel  lines  by  breaking. up,  on 
account  of  sintering,  of  the  fibers  composing  the  raw 
gypsum;  cut  No.  13  shows  the  grains  in  irregular 
position  and  distinctly  separated  one  from  another. 
The  samples  burnt  at  1000  and  1100  centigrade  differ 
from  those  obtained  at  900  centigrade  only  by  the 
larger  dimensions  of  the  grains. 

Chemical  Composition  of  Hydraulic  Gypsum:  The 
erroneous  assumption  thai  ^hydraulic  gypsum  was 
formed  at  temperatures  between  400  and  600  centi- 
grade, within  which  interval  no  chemical  change  in 
the  composition  of  the  gypsum  could  be  discovered 
to  take  place,  outside  of  giving  off  its  water  of 
crystallization,  induced  chemists  to  declare,  that 
hydraulic  gypsum  was  "a  second  anhydrous  modifi- 
cation of  gypsum"  and  that  it  had  the  same  compo- 
sition as  the  slow-setting  I.  anhydrite  obtainable  be- 
tween 200  and  300  centigrade.  Yet,  in  making  this 
statement  it  was  overlooked  that  the  hydraulic  gyp- 
sum, as  supplied  by  the  manufacturer,  invariably  con- 
tains more  calcium  oxide  than  corresponds  with  the 
amount  of  sulphuric  acid  present.  This  surplus  of 
calcium  oxide  may  originate  from  two  causes:  It  may 
derive  from  the  decomposition  of  calcium  carbonate, 
if  the  native  gypsum  employed  in  the  manufacture 
contains  some  of  this,  or  it  may  be  due  to  a  partial 
evaporation  of  the  sulphuric  acid  of  the  gypsum. 
Now,  a  similar  partial  decomposition  of  gypsum  takes 
place,  according  to  my  experiments,  at  a  temperature 
of  800  centigrade,  that  is  to  say  at  the  minimum  tem- 
perature necessary  for  the  formation  of  the  granular 
modification  of  the  anhydrite,  and  must  be  con- 
sidered inseparable  from  the  burning  of  hydraulic 
gypsum.  The  surplus  of  calcium  oxide  grows  with 
the  increase  of  the  burning  temperature  as  the  fol- 
lowing table  illustrates: 

Chemical  Composition  of  Burnt  Product  in  Per  Cent. 
Temperature  Calcium  Sulphuric  equal  CaSO*  +  CaO 
In  Centigrade  Oxide  Acid  to 

800  42.64  57.02  9693          2.73 

i  loo  43.06  56.40  9588         3.58 

1400  47.72  51.88  88.20        1140 

29 


This  "free''  calcium  oxide,  originating  from  a  par- 
tial decomposition  of  gypsum,  behaves,  during  the 
hardening  of  hydraulic  gypsum,  in  an  entirely  dif- 
ferent way  from  free  calcium  oxide  derived  from 
calcium  carbonate,  as  my  experiments  prove.  Also 
micro-photographs  show  a  marked  difference.  A  mix^ 
ture  of  2  parts  of  gypsum  and  1  part  of  finely  pul- 
verized chalk,  burnt  at  a  temperature  of  1070  centi- 
grade, presented  under  the  microscope  the  minute 
grains  of  calcium  oxide  between  the  much  larger 
grains  of  hydraulic  gypsum,  which  latter  also  en- 
closed grams  of  calcium  oxide.  No  action  whatever 
of  one  constituent  upon  the  other  was  revealed  by 
any  change  of  form  which  might  possibly  have  taken 
place.  In  accordance  with  this  observation,  the 
pulverized  calcined  product  developed  considerable 
heat  upon  being  mixed  with  water  and  resulted,  after 
having  hardened,  in  a  chalk-like  mass  of  low  strength, 
that,  by  the  way,  did  not  show  signs  of  blowing, 
because  the  calcium  oxide  hydrated  immediately 
upon  mixing  of  the  powder  with  water. 

On  the  other  hand,  the  calcined  products  of  pure 
raw  gypsum  did  not  show  any  signs  whatever  of  free 
calcium  oxide,  not  even  the  sample  burnt  at  1400 
centigrade,  and  all  of  them  showed  normal  harden- 
ing. The  hydraulic  gypsum  burnt  at  1400  centigrade 
developed  even  a  remarkably  high  strength.  Also  a 
sample,  placed  in  a  furnace  in  which  china  was  burnt 
at  1370  centigrade  and  which,  on  account  of  too  long 
exposure  to  the  fire,  contained  only  17%  of  calcium 
sulphate  and  83%  of  "free"  calcium  oxide,  hardened 
well  and  exhibited  no  signs  of  blowing. 

The  next  question  occuring  to  one's  mind  is:  In 
What  Form  is  the  Calcium  Oxide  Contained  in  Hy- 
draulic Gypsum?  To  this  a  satisfactory  and  at  the 
same  time  surprising  answer  was  obtained  from  a 
study  of  the  microscopic  sections  of  the  samples 
burnt  at  higher  temperatures,  for  instance  at  1300 
centigrade.  As  a  result  it  must  be  stated  that  hy- 
draulic gypsum  is  by  no  means  a  homogeneous  body, 
as  was  the  general  belief  so  far;  it  consists  of  two 
components,  in  the  first  place  of  the  above  mentioned 
grains  or  polygons  and  in  the  second  place  of  a  glassy 
substance  surrounding  the  former.  The  granular 

30 


constituent  is,  as  formerly  described,  undoubtedly 
crystalline  and  double-refracting,  the  latter  sub- 
stance, in  distinction  thereof,  amorphous  and  from  an 
optical  standpoint  inactive.  On  microscopic  sections 
(see  cut  No.  14)  the  glass  substance  appears  as  net- 
work enclosing  in  its  meshes  the  grains  or  polygonal 
bodies. 

There  is  no  doubt  that  the  "free"  calcium  oxide  of 
hydraulic  gypsum  is  contained  in  the  glassy  sub- 
stance, while  the  grains  represent  a  special  granular 
modification  of  the  anhydrite  of  calcium  sulphate. 
This  follows  from  the  quantitative  proportions  of  the 


No.  14.  Hydraulic  gypsum  burnt  from  alabas- 
ter gypsum  at  1300  centigrade.  The  grains  of 
anhydrite  lie  in  a  network  of  a  fused  glassy 
"basic  calcium  sulphate."  In  comparing  the  pho- 
tograph of  this  preparation  with  Nos.  12  and  13  it 
must  be  remembered  that  it  has  been  enlarged  only 
half  as  much  as  the  latter.  The  grains  are  much 
larger  in  this  sample  of  hydraulic  gypsum  on 
account  of  the  higher  burning  temperature.  Mag- 
nified 375  times. 

31 


"free"  calcium  oxide  to  calcium  sulphate  in  the  burnt 
products  and  from  those  of  the  glass  substance  to 
the  granular  part  in  the  corresponding  micro-photo- 
graphs. The  commercial  hydraulic  gypsum,  which  is 
burnt  at  a  temperature  between  900  and  1000  centi- 
grade, contains  on  an  average  3%  of  "free"  calcium 
oxide,  unless  the  raw  gypsum  contained  calcium  car- 
bonate. The  glass  substance,  therefore,  becomes 


ho.  15.  Microscopic  section  of  hydraulic  gyp- 
sum burnt  at  1400  centigrade,  showing  characteris- 
tic plates  of  sintered  anhydrite.  The  dark  parts  re- 
present glassy  "basic  calcium  sulphate."  Magnified 
loo  times. 


hardly  visible  and  can  be  distinguished  as  network 
only  upon  powerful  magnification.  My  attention  was 
called  to  it  for  the  first  time  by  sections  secured  from 
samples  burnt  at  higher  temperatures  which  con- 
tained larger  amounts  of  "free"  calcium  oxide.  In 
these  the  glass  substance  became  plainly  noticeable 
even  if  magnified  moderately. 


The  "free"  calcium  oxide  is  evidently  an  important 
part  of  hydraulic  gypsum ;  it  belongs  to  its  chemical 
constitution  and  must  not  be  regarded  as  adultera- 
tion or  occasional  admixture  as  done  heretofore. 
However,  it  is  not  contained  in  it  in  a  free,  uncom- 
bined  state  in  the  usual  sense  of  the  word,  as  it  can 
not  be  detected  by  the  microscope.  The  glassy  sub- 
stance, therefore,  (which  has  merely  been  termed 
thus  on  account  of  its  transparency  and  not  because 
it  was  supposed  to  resemble  glass  from  a  chemical 
standpoint)  must  either  be  a  basic  calcium  sulphate 
or  must  represent  a  solution  of  calcium  oxide  in 
neutral  calcium-sulphate, 

Theory  of  the  Formation  of  Hydraulic  Gypsum: 
The  results  obtained  from  the  foregoing  observations 
explain  the  chemical  processes  going  on  during  the 
burning  of  hydraulic  gypsum.  With  increasing 
temperature  the  raw  gypsum  passes  through  the 
stages  of  the  half-hydrate,  of  the  quick-setting  and 
slow-setting  I.  anhydrite  and  as  continuation  of  the 
latter  that  of  the  overburnt  gypsum.  The  point, 
when  the  calcium  sulphate  begins  to  decompose, 
under  partial  evaporation  of  the  sulphuric  acid,  is  at 
the  same  time  the  beginning  of  the  glassy  formation 
of  "basic  calcium  sulphate"  and  of  the  granular  modi- 
fication of  anhydrite.  This  takes  place  at  a  tem- 
perature of  about  800  centigrade.  The  grains  are 
still  exceedingly  small  at  this  temperature.  Upon 
increasing  the  temperature  to  900  centigrade  and 
upon  further  evaporation  of  small  amounts  of  sul- 
phuric acid,  which  means  also  further  formation  of 
the  glassy  component,  the  grains  obtain  the  normal 
size  of  the  commercial  hydraulic  gypsum.  If  the 
burning  temperature  is  further  increased,  the  glassy 
"basic  calcium  sulphate"  assumes  even  larger  propor- 
tions and  the  grains  of  anhydrite  become  still  larger 
until,  at  about  1400  centigrade,  near  the  fusing  point 
of  the  mass,  a  coarse  crystalline  product  is  obtained 
composed  of  large  amounts  of  plates  and  tablets  of 
anhydrite  in  addition  to  the  customary  round  or 
polygonal  grains.  These  plates  obtain  a  length  and 
width  of  1  millimeter  and  more  and  are  divided  in 
smaller  areas  by  shrinking  cracks  crossing  at  right 
angles.  Cut  No.  15  shows  a  similar  plate;  the  glassy 


"basic  calcium  sulphate"  partly  fills  out  the  cracks, 
partly  it  covers  the  plate  at  a  number  of  places. 
Where  it  predominates,  it  often  encloses  grains  of 
anhydrite  on  the  point  of  decomposition. 

The  genuine  commercial  product  of  hydraulic  gyp- 
sum contains  all  sizes  and  forms  of  anhydrite,  from 
the  minute  grain  formed  at  800  centigrade  up  to  the 
large  plates  coresponding  with  temperatures  of  1300 
and  1400  centigrade.  The  most  frequent,  however, 
are  the  round  or  polygonal  grains  of  medium  size 
which  are  ^principally  formed  between  900  and  1000 
centigrade.  This  shows  that  the  temperature  differs 
widely  in  the  same  furnace,  which  can  not  be  avoided, 
of  course,  if  the  charge  of  the  kiln  consists  of  alternate 
layers  of  raw  gypsum  and  coal.  Fortunately  the 
quality  of  the  calcined  product  is  not  impaired  by  it, 
as  all  grades  of  hydraulic  gypsum  burnt  between  800 
and  1400  centigrade  harden  well.  A  matter  of  great 
importance,  however,  is  that  the  burning  is  done 
with  an  oxidizing  flame.  A  reducing  atmosphere 
causes  formation  of  calcium  sulphide  and  decreases 
the  value  of  the  burnt  product. 

Setting  and  Hardening  of  Hydraulic  Gypsum: 
The  statements  to  be  found  in  the  literature  on  gyp- 
sum to  the  effect  that  the  hardening  of  hydraulic 
gypsum  was  due  to  the  going  into  solution  of  the 
anhydrite  and  re-crystallizing  as  di-hydrate  from  the 
over-saturated  solution,  were,  as  pointed  out  in  the 
foregoing,  based  upon  observations  made  on  slow- 
setting  plaster  and  not  on  hydraulic  gypsum.  The 
explanation  given  was  not  so  incredible,  as  the  hard- 
ening process  of  plaster  of  Paris  could  readily  be 
believed  to  be  applicable  also  to  hydraulic  gypsum. 
The  slower  hardening  of  the  latter  was  simply  at- 
tributed to  its  decreased  solubility  in  water  and  to  its 
reduced  ability  to  form  oversaturated  solutions. 

However,  the  observations  to  be  made  during  the 
practical  application  of  hydraulic  gypsum  strongly 
discredit  these  views.  A  hardening  process,  as  that 
of  plaster  of  Paris,  caused  by  interlacing  and  grow- 
ing together  of  crystals  of  di-hydrate,  appears  in- 
feasible  in  the  case  of  hydraulic  gypsum,  which 
requires  pounding  with  wooden  mallets  and  smooth- 
ing with  trowels  after  the  gypsum  has  set  for  a  day 

34 


or  two,  in  order  to  give  it  its  maximum  density  and 
strength.  If  the  hardening  were  caused  by  the 
formation  of  crystals,  a  similar  treatment  would  be 
of  no  avail,  on  the  contrary  it  would  be  highly  in- 
jurious, because  the  crystals  would  be  crushed  one 
by  another  and  the  cohesion  be  destroyed.  Moreover, 
it  would  be  inexplainable  why  hydraulic  gypsum  pos- 
sesses far  greater  hardness  and  superior  strength  to 
plaster  of  Paris  and  slowsetting  plaster,  if  the  phys- 
ical and  chemical  properties  of  the  hardened  products 
were  identical,  namely  the  same  crystallized  di- 
hydrate. 


No.  16.  Section  through  a  hardened  piece  of  hy- 
draulic gypsum  showing  the  unchanged  position  of 
its  component  parts  due  to  the  customary  coarse 
grinding.  Magnified  750  times. 


It  was  mentioned  in  the  foregoing  that,  according 
to  my  observations,  powdered  hydraulic  gypsum 
placed  in  water  remained,  in  contrast  to  plaster  of 
Paris,  entirely  unchanged  for  many  months  and  did 
not  show  the  slightest  inclination  to  form  crystals. 

35 


This  observation  alone  would  have  been  sufficient  to 
suggest  a  quite  different  process  of  hardening;  and, 
in  fact,  microscopic  sections  of  thoroughly  hardened 
hydraulic  gypsum  show  hardly  a  trace  of  crystalliza- 
tion. The  component  parts  of  the  original  calcined 
product  are  found  unaltered  in  the  hardened  gypsum 
as  cuts  No.  16  and  17  prove. 

Only  now  and  then  hollow  spaces  can  be  detected 
partly  filled  out  by  crystals  of  di-hydrate  (see  cut  No. 
18).  Crystallization,  therefore,  is  of  secondary  im- 
portance in  the^  case  of  hydraulic  gypsum.  Hardened 
pulverized  hydraulic  gypsum  can  be  distinguished 
from  the  unhydrated  calcined  product  only  by  the 
more  regular  alignment  of  the  grains  of  the  latter. 
But,  owing  to  the  fact,  that  this  gypsum  is  mostly 
sold  as  a  coarse  powder,  the  hardened  gypsum  repre- 
sents in  most  cases  a  conglomerate  of  pieces  showing 
the  granular  elements  in  an  unchanged  position. 

Hence,  hydraulic  gypsum  hydrates  principally 
without  changing  its  form,  without  crystallizing,  be- 
cause it  is  unable  to  form  over-saturated  solutions. 
This  explains  the  necessitv  of  densifying  the  mortar 
during  the  early  stage  of  its  hardening  bv  tamping 
it ;  the  particles  are  thus  forced  together  more  closely : 
hereby  the  surfaces  of  contact  increase  and  the  grains 
are  cemented  more  completely.  The  tamping  of  the 
mortar,  however,  one  or  two  days  after  hardening 
has  beg^un,  seems  to  be  unavoidable,  as  the  freshlv 
gauged  gypsum  is  too  soft  to  withstand  tamping.  It 
would  be  better,  of  course,  if  such  a  disturbance  of 
the  setting  process  could  be  avoided;  yet,  it  is  of  lit- 
tle consequence  in  as  much  as,  at  the  time  of  tamp- 
ing, the  process  of  hydration  is  still  in  its  incipiency 
and  as  only  a  small  amount  of  water  has  gone  into 
combination.  A  determination  of  the  water  of  com- 
bination of  a  piece  of  hardening  hydraulic  gypsum 
showed  2.18%  after  2  days,  5.60%  after  14  days  and 
12.0%  after  11  weeks,  while  about  21%  were  required 
for  complete  transformation  into  the  hydrated  state. 
This  proves  the  importance  of  keeping  hardening 
hvdraulic  gypsum  wet  for  several  months,  if  it  is  ex- 
pected to  attain  its  maximum  strength.  It  closely 
resembles  Portland  cement  in  this  respect. 

While     the     granular     constituent     of     hydraulic 

36 


gypsum,  the  anhydrite,  is  converted,  upon  hardening, 
into  di-hydrate  without  change  of  form,  that  is  to  say 
without  crystallizing,  the  glassy  component  may  take 
a  share  in  the  hardening  in  a  two-fold  manner.  In  the 
first  place,  it  is  not  at  all  impossible,  that  its  "free" 
calcium  oxide,  contained  in  it  in  "solid  solution," 
upon  coming  in  contact  with  water,  crystallizes  as 


No.  17.  Section  through  hardened  hydraulic  gyp- 
sum. In  this  case  the  calcined  product  was  ground 
more  finely.  The  grains,  therefore,  do  not  appear 
in  rows,  but  are  isolated  and  mixed.  Magnified  750 
times. 


calcium  hydrate.  But,  this  observation  I  made  only 
in  the  case  of  the  product  calcined  in  a  furnace 
together  with  china,  which,  with  its  Yt%  of  calcium 
sulphate  and  83%  of  calcium  oxide,  represented  a  very 
basic  glass-like  substance.  Its  powder  was  acted 
upon  very  strongly  by  water;  however,  there  was  no 
slaking  or  disintegrating  noticeable ;  the  process  con- 
sisted in  a  lively  formation  of  small  and  large  plate- 

37 


crystals  of  calcium  hydrate  belonging  apparently  to 
the  hexagonal  system.  In  the  case  of  commercial 
hydraulic  gypsum,  with  approximately  3%  of  "free" 
calcium  oxide,  I  never  observed  similar  formation  of 
crystals  of  calcium  hydrate  upon  adding  water,  nor 
did  I  detect  them  in  hardened  pieces  of  hydraulic 
gypsum. 


No.  18.  Hollow  spaces  in  a  piece  of  hardened 
hydraulic  gypsum  containing  a  few  crystals  of  di- 
hydrate.  These  crystals  are  due  to  the  evapora- 
tion of  the  water  of  the  saturated  (not  over-sat- 
urated) solution  of  calcium  sulphate  that  formerly 
filled  out  these  cavities. 


The  second  possibility  of  contributing  to  the  hard- 
ening for  the  "basic  calcium  sulphate/'  and  which  is 
more  likely  to  be  the  true  explanation,  immediately 
suggests  itself  by  the  going  into  solution  of  calcium 
hydrate  upon  pouring  hydraulic  gypsum  into  a  large 
amount  of  water.  The  water  becomes  strongly  alka- 
line after  a  short  time  and  the  surface  of  it  soon  ap- 
pears covered  with  a  thin  film  consisting  of  minute 

88 


calcspar-crystals.  This  process  of  going  into  solution 
of  calcium  hydrate  and  precipitating  by  the  carbonic 
acid  of  the  atmosphere  undoubtedly  takes  place  also 
during  the  hardening  of  hydraulic  gypsum  as  long  as 
"basic  calcium  sulphate"  is  available.  The  process  is 
the  same  as  that  causing  the  hardening  of  ordinary 
lime  mortar,  but  of  minor  importance  in  the  case  of 
hydraulic  gypsum.  The  minute  calcspar-crystals  can 
be  detected  in  hardened  hydraulic  gypsum  only  by 
the  development  of  carbonic  acid,  if  a  drop  of  hydro- 
chloric acid  is  allowed  to  act  on.  a  microscopic  sec- 
tion. After  the  "free"  calcium  oxide  has  been  ex- 
tracted from  the  "basic  calcium  sulphate,"  the  re- 
maining neutral  calcium  sulphate  hydrates  in  the 
same  way  as  the  granular  anhydrite,  that  is  to  say 
without  forming  crystals,  as  all  sections  of  hardened 
hydraulic  gypsum  prove. 

The  third  factor  coming  in  question  in  the  harden- 
ing of  hydraulic  gypsum,  and  likewise  a  point  of 
little  consequence,  is  its  solubility  in  water,  which 
lies  between  that  of  the  half-hydrate  and  of  the  di- 
hydrate  and  which  certainly  surpasses  that  of  native 
gypsum.  The  small  amount  of  calcium  sulphate 
going  into  solution  may  be  sufficient  to  partly  or  en- 
tirely fill  out  with  small  crystals  of  di-hydrate  some 
of  the  hollow  spaces  created  by  the  gradual  drying 
out  of  the  hardened  gypsum.  This  feature  presents 
cut  No.  18. 

A  point  of  greater  importance  for  its  technical  ap- 
plication is  a  property  characteristic  for  the  grains 
of  hydraulic  gypsum,  namely  that  they  retain  the 
hardness,  which  they  obtained  in  the  calcining  pro- 
cess, also  after  they  have  combined  with  water.  This 
forms  the  main  cause  of  the  strength  of  hardened 
hydraulic  gypsum.  Such  strength  would  be  out  of 
the  question  if  the  hardening,  as  assumed  so  far,  would 
be  due  to  transformation  into  crystals  of  di-hydrate. 

Influence  of  Catalysers  Upon  Hydraulic  Gypsum: 
P.  Rohland  states  that  the  hardening  process  of 
hydraulic  gypsum  can  be  accelerated  by  an  admixture 
of  positive  catalysers ;  thus  he  found  the  hardening 
time  reduced  from  600  to  60  hours  by  an  addition  of 
potassium  sulphate.  If  this  were  true,  it  would  seem 

39 


to  be  of  some  benefit  in  the  practical  application  of 
hydraulic  gypsum. 

With  the  aim  to  throw  light  on  this  point  I  made 
a  series  of  experiments,  especially  with  potassium 
aluminum  sulphate.  These  tests  prove  that  hydraulic 
gypsum,  gauged  with  *  saturated  solutions  of  alum, 
precipitates  needle-crystals  of  di-hydrate  as  early  as 
15  or  20  minutes  after  being  mixed  with  the  solution. 
After  2  hours  the  small  grains  are  completely  con- 
verted into  crystals.  After  12  hours  about  half  of  it 
and  after  24  or  30  hours  almost  the  whole  original 
mass  is  crystallized.  The  process  is  identically  the 
same  as  if  "overburnt"  gypsum  is  used,  which  was 
burnt  at  temperatures  between  300  and  700  centi- 
grade. 

The  application  of  accelerating  admixtures,  there- 
fore, undoubtedly  shortens  the  hardening  of  hydraulic 
gypsum,  but  the  hardened  product  obtained  is  no 
longer  hydraulic  gypsum;  it  corresponds  in  every 
respect  with  a  gypsum  burnt  at  500  centigrade  and 
subsequently  treated  with  alum  solution;  owing  to 
the  formation  of  larger  crystals,  the  edges  of  the 
hardened  mortar  are  translucid  and  the  whole  casting 
is  considerably  stronger  than  hardened  plaster  of 
Paris;  yet,  microscopic  examination  reveals  at  a 
glance,  that  this  is  an  entirel^  different  hardened  pro- 
duct from  hydraulic  gypsum  hardened  with  pure 
water. 


40 


IV.    SUMMARY  OF  THE  RESULTS  AND  CON- 
CLUSIONS  DRAWN   FROM   THEM  WITH 
REGARD  TO  PRACTICAL 
APPLICATION. 

By  the  foregoing  observations  has  been  proven 
that  almost  all  former  statements  about  and  concep- 
tions of  the  origin,  constitution  and  properties  of 
hydraulic  gypsum  to  be  found  in  modern  literature 
are  erroneous  and  that  this  chapter  of  technology  has 
to  be  revised  completely  in  order  to  make  it  agree 
with  my  observations  and  with  the  practical  experi- 
ence of  the  manufacturer.  The  general  opinion  that 
hydraulic  gypsum  is  obtained  at  temperatures  be- 
tween 400  and  600  centigrade  and  even  still  lower 
has  to  be  positively  abandoned.  Furthermore,  hy- 
draulic gypsum  is  by  no  means  identical  with  the 
soluble  anhydrite ;  it  does  not  show  the  needle-crystals 
of  di-hydrate  as  plaster  of  Paris  and  the  lower 
grades  of  "overburnt"  gypsum,  slow-setting  plaster, 
do.  In  opposition  hereto,  the  results  obtained  from 
mv  research-work  characterize  hydraulic  gypsum  as 
follows : 

Hydraulic  gypsum  consists  of  a  mixture  of  a  gran- 
ular, densely  vitrified,  hard,  strongly  light-refract- 
ing, crystalline  modification  of  the  anhydrite,  ob- 
tained at  red  heat,  with  small  amounts  of  a  fused, 
amorphous,  glassy,  basic  anhydrite,  containing  cal- 
cium oxide  in  "solid  solution."  This  product  requires 
for  its  formation  and  for  the  complete  development 
of  its  constituents  a  temperature  of  about  900  centi- 
grade. At  higher  temperatures,  at  1300  centigrade 
and  above,  the  amount  of  the  basic  constituent  in- 
creases and  the  grains  grow  in  size  and  hardness.  It 
combines  with  water  without  change  of  shape,  that 
is  to  say  without  forming  crystals,  with  the  excep- 
tion of  very  small  amounts  of  crystallized  di-hydrate 

41 


and  calcium  hydrate,  and  retains  its  optic  properties 
and  hardness.  Positive  catalysers,  however,  trans- 
form it  into  soluble  anhydrite  and  crystals  of  di- 
hydrate. 

For  the  practical  gypsum  burner  the  most  im- 
portant and  noteworthy  point  is  this,  that  it  is  al- 
most out  of  the  question  to  spoil  a  charge  by  over- 
burning.  The  heat  may  be  increased  to  1300  centi- 
grade and  even  higher,  but  not  for  too  long  a  time, 
without  impairing  the  hardening  properties,  provid- 
ing the  firejs  conducted  in  such  a  way  that  the  gyp- 
sum is  not  reduced  to  calcium  sulphide.  On  the  other 
hand,  the  temperature  may  go  down  as  low  as  800 
centigrade;  however,  it  will  always  be  advisable  to 
regard  900  centigrade  as  the  lowest  limit,  because 
only  then  the  characteristic  hard-sintered  grain  is 
formed.  The  wide  interval  of  from  400  to  500  de- 
grees, within  which  the  heat  in  the  kiln  may  vary, 
makes  the  burning  process  very  convenient  and  safe. 
Heating  above  1000  centigrade  is  not  economical,  of 
course;  yet,  I  wish  to  repeat  at  this  place,  that  at 
very  high  temperatures,  between  1300  and  1400  centi- 
grade— at  least  in  laboratory  experiments — a  still 
coarser  and  apparently  even  harder,  because  more 
strongly  sintered,  grain  is  obtained,  which  may  pos- 
sibly be  of  value  for  some  purposes.  But  as  not  only 
the  height  of  the  temperature,  but  also  its  duration, 
governs  the  physical  properties  of  the  grain,  the  same 
effect  is  likelv  to  be  obtained  also  at  lower  tempera- 
tures in  practical  operation. 

By  my  research  has  been  demonstrated,  from  a 
scientific  standpoint,  that  hydraulic  gypsum  repre- 
sents a  product  differing  in  every  respect  from 
plaster  of  Paris  and  slow-setting  plaster,  which  the 
manufacturer  claimed  to  be  the  case  for  some  time. 
It  is  a  more  precious  substance,  the  superior  qualities 
of  which  are  certainly  not  sufficiently  appreciated  as 
yet  by  the  building  trade.  For  many  years,  for 
instance,  vain  efforts  have  been  made  to  produce  a 
white  cement,  until  at  last  a  number  of  factories  suc- 
ceeded in  turning  out  a  fairly  satisfactory  but  expen- 
sive product.  Hydraulic  gypsum  is  a  white  cement 
of  the  highest  order.  On  account  of  its  excellent  hy- 
draulic properties,  being  fully  equal  to  those  of  Port- 

42 


land  cement,  and  because  of  its  dense  structure,  which 
minimizes  the  danger  of  soiling  of  the  surface,  this 
cement  is  surely  superior  to  white  Portland  cement 
and  should  be  ttsed  more  extensively. 

Among  other  uses,  it  would  be  advisable,  for  in- 
stance, to  try  hydraulic  gypsum  for  castings  of  plastic 
works  of  art  and  of  architectural  decorations.  To 
this  suggestion  the  objection  may  be  made  that 
hydraulic  gypsum  is  mostly  too  coarsely  ground  for 
this  purpose  and  that  it  is  heavier  than  plaster;  more- 
over, that  it  does  not  set  as  rapidly.  There  is  nothing 
in  the  way  of  grinding  it  more  finely;  on  the  con- 
trary, this  would  be  an  easy  matter,  as  the  calcined 
gypsum  shows  a  natural  inclination  of  breaking  up 
into  the  minute  grains  composing  it.  The  tamping 
of  such  castings,  a  day  or  two  after  having  been 
poured  into  the  moulds,  would  have  to  be  avoided 
most  likely.  Therefore,  another  method  would  have 
to  be  employed  in  order  to  provide  greatest  possible 
density.  This  could  be  achieved  by  mixing  two 
kinds  of  hydraulic  gypsum  burnt  at  different  tem- 
peratures and  ground  to  different  fineness,  for  in- 
stance, coarsely  ground  hydraulic  gypsum  burnt 
between  1200  and  1300  centigrade,  with  its  large 
granular  elements,  and  finely  ground  hydraulic  gyp- 
sum calcined  at  about  800  centigrade.  The  minute 
grains  of  the  latter  would  fill  out  all  voids  between 
the  grains  of  the  former.  Such  a  process  of  mixing 
would  be  based  upon  the  experience,  that  the  size  of 
the  granular  constituents  of  hydraulic  gypsum  grows 
with  increased  heat  and  would  soon  lead  to  a  mixture 
possessing  the  greatest  possible  weight  for  a  given 
volume.  Experiments  made  in  this  direction  showed 
a  weight  of  149  grams  per  100  ccm.  of  hydraulic 
gypsum  passing  a  100-mesh  sieve  and  of  190  grams 
for  the  same  volume  of  hydraulic  gypsum  passing 
the  200-mesh  sieve.  Of  the  various  mixtures  made 
from  these  two  screenings  that  showed  the  highest 
weight  per  given  volume  (100  ccm.),  namely  203 
grams,  which  contained  40%  of  the  coarse  and  60% 
of  the  fine  product.  For  this  test  only  one  kind  of 
hydraulic  gypsum  was  used  possessing  granular  ele- 
ments of  almost  even  size.  If  two  products  of  widely 
differing  burning  temperature  would  have  been  used, 

43 


a  still  higher  weight  by  volume  would  have  been  ob- 
tained undoubtedly. 

The  slow  progress  made  in  the  hardening  of  such 
castings  would  be  a  drawback,  of  course;  yet,  their 
strength  after  a  few  days  is  sufficient  to  allow  taking 
them  out  of  the  moulds.  Afterwards  they  have  merely 
to  be  kept  in  a  chamber  saturated  with  moisture  and  to 
be  sprinkled  with  water  now  and  then  in  order  not 
to  interrupt  the  hardening  process  by  drying  out  of 
the  castings.  If  the  moulds  have  to  be  used  again 
very  soon,  the  hardening  of  hydraulic  gypsum  may 
be  accelerated  by  small  admixtures  (about  10%  of  its 
weight)  of  slow-setting  plaster,  burnt  between  200 
and  220  centigrade.  A  similar  plaster  hardens  more 
slowly  than  ordinary  plaster  of  Paris,  but  sets  suf- 
ficiently fast  and  has  this  advantage  over  the  latter, 
that  it  forms  larger  crystals  of  di-hydrate.  On  the 
other  hand,  the  strength  and  durability  of  plaster  of 
Paris  could  be  improved  by  an  admixture  of  hydraulic 
gypsum. 

Microscopic  Determination  of  the  Commercial  Pro- 
ducts of  Gypsum:  On  account  of  the  present  uncer- 
tainty, or  even  confusion,  prevailing  in  the  definition 
of  hydraulic  gypsum  and  on  account  of  its  frequent 
being  mistaken  for  the  slow-setting  "soluble"  or  first 
anhydrite  (burnt  between  200  and  350  centigrade) 
and  even  for  plaster  of  Paris,  a  simple,  reliable  and 
brief  determination  of  the  various  products  manu- 
factured from  native  gypsum  would  certainly  be  de- 
sirable and  of  great  value.  The  microscopical  exami- 
nation enables  the  investigator  to  attain  this  end 
speedily  and  safely.  Genuine  hydraulic  gypsum,  that 
is  to  say  gypsum  calcined  at  a  temperature  of  at  least 
900  centigrade,  can  be  easily  distinguished  through  it 
from  all  other  kinds  of  burnt  gypsum  and  also  from 
powdered  raw  gypsum,  so  that  this  method  should 
be  employed  in  all  doubtful  cases.  A  similar  investi- 
gation requires  but  a  few  minutes.  A  trace  of  the 
powder  to  be  examined  is  stirred  with  a  drop  of 
water  on  a  miscroscopic  slide  and  a  cover  glass  is 
placed  upon  it.  Then  the  cover  glass  is  subjected  to 
a  slight  pressure  in  order  to  disintegrate  the  coarser 
grains  .and  to  make  the  conglomerations  of  the  gran- 
ular constituents  of  hydraulic  gvpsum  separate  into 

44 


their  elements,  grains  or  plates  or  whatever  the  case 
may  be.  The  preparation  should  be  magnified  300  or 
400  times;  but  even  200-fold  magnification  suffices  in 
most  instances. 

In  analyzing  the  fragments  to  be  observed  under 
the  microscope  the  following  must  be  borne  in  mind: 
All  particles  of  fibrous  and  translucid  appearance  and 
those  of  a  yellowish-brown  color  are  burnt  below 
800  centigrade  and  have  nothing  in  common  with 
hydraulic  gypsum.  They  have  the  shape  of  prongs, 
spikes  or  splinters  and  contain  numerous  needle- 
shaped  fragments  deriving  from  the  crystallized 
fibers  of  the  original  raw  gypsum.  Cut  No.  11  and 
No.  8  illustrate  these  fragments.  In  opposition  here- 
to, the  microscopic  elements  of  hydraulic  gypsum 
show  not  the  least  sign  of  a  fibrous  structure  (with 
the  exception  of  that  burnt  at  800  centigrade,  the 
temperature  at  which  the  formation  of  hydraulic  gyp- 
sum begins).  They  are  entirely  clear  and  transparent 
in  almost  every  instance  and,  owing  to  their  strong 
light-refraction,  appear  lighter  than  the  surrounding 
water.  The  grains  and  rows  of  granular  elements 
possess  forms  so  characteristic  (  as  cut  No.  10  and  No. 
12-17  show),  that  a  particle  of  hydraulic  gypsum, 
which  happens  to  be  in  plaster  of  Paris,  is  detected 
at  a  glance.  Needle-crystals  or  fragments  of  them 
do  not  occur  in  hydraulic  gypsum  burnt  at  900  centi- 
grade and  above,  so  that  the  absence  of  them  is  a 
sure  sign  of  genuine  hydraulic  gypsum.  Another 
characteristic  for  hydraulic  gypsum  are  the  minute 
holes  often  found  in  the  granular  elements.  In  polar- 
ized light  the  particles  of  hydraulic  gypsum  can  be 
readily  distinguished  from  all  kinds  of  gypsum  burnt 
below  800  centigrade  by  their  remarkably  strong  iri- 
descence. 

The  microscope,  however,  furnishes  still  further 
information  about  hydraulic  gypsum.  It  permits  to 
determine  approximately  its  burning-  temperature  on 
account  of  the  growing  size  of  its  constituents  with 
rising  temperatures.  Large  grains  indicate  a  high 
burning  temperature,  while  rectangular  plates  (see 
cut  No.  15)  or  fragments  of  them  prove  that  the  pro- 
duct was  calcined  at  a  temperature  slightly  below  its 
fusing  point.  In  connection  with  these  views,  pre- 
45 


sented  by  the  microscope,  it  must  be  remembered  that 
the  hardness  of  the  grain  and  consequently  the 
strength  of  the  resulting  mortar  grow  with  the  size 
of  the  granular  component  of  hydraulic  gypsum. 

Plaster  of  Paris  can  be  distinguished  from  the  slow- 
setting  i.  anhydrite  oy  the  time  when  crystallisation 
of  di-hydrate  begins.  The  exterior  shape  of  their 
particles  offers  no  marks  of  distinction. 

In  conclusion  the  various  calcined  products  obtain- 
able from  Jaw  gypsum  may  be  classified  in  accordance 
with  the  results  derived  from  the  foregoing  research. 
The  limits  of  temperatures  stated  must  only  be  con- 
sidered as  approximate  figures,  of  course,  as  the 
change  from  one  kind  to  the  other  takes  place  very 
gradually  and  because,  as  repeatedly  stated,  not  only 
the  height  of  the  temperature,  but  also  its  duration, 
determine  the  properties  of  the  calcined  product: 

A.  Native  Gypsum — Di-hydrate,  containing  2  mole- 

cules of  water. 

B.  107  Celsius— Half-hydrate,     containing     y2 

mol.  of  water. 

C.  107-170         "       — Consists    mainly    of    half-hyd- 

rate. 

D.  170-200         "       —More  or  less  dehydrated  half- 

hydrate.  Combines  with  wat- 
er readily  until  half-hydrate 
is  obtained. 

C.  &  D.  represent  the  commer- 
cial plaster  of  Paris. 

E.  200-250  —Contains  a  very  small  amount 

of  water.    Sets  more  slowly 
than  the  former.     . 

F.  250-400         "       — Contains  only  a  trace  of  water. 

Slow-setting. 

B.,  C.,  D.,  E,  &  F.  form  crys- 
stals  of  di-hydrate,  if  mixed 
with  water.  Hardening  due 
to  crystallization. 

G.  400-750  —Completely     dehydrated,     an- 

hydrite, over-burnt  from  a 
practical  point  of  view. 

H.       750-800         "      —Gradual    transformation     into 

the  granular  modification  of 

46 


anhydrite;  beginning  of  the 
formation  of  hydraulic  gyp- 
sum. 

G.  &  H.    Show,    in    contact 
with   water,   no   hardening 
or  only  very  imperfect  hard- 
ening. 
I.       800  "       — Hydraulic   gypsum,    containing 

minute  grains  of  anhydrite. 
K.       900-1000       "      —Genuine     hydraulic     gypsum; 

grains  fully  developed. 

L.     1000-1400       "      — Hydraulic     gypsum,     showing 

grains  increasing  in  size  and 
hardness  with  rising  temper- 
ature. The  percentage  of 
"basic  calcium  sulphate"  like- 
wise increases  in  the  same 

ratio. 

I.,  K.  &  L.,  harden  slowly 
with  water  without  crystal- 
lizing. 

G.,  H.,  L,  K.,  &  L.  crystallize 
with  alum  solution. 

A  temperature  of  from  1300  to  1400  centigrade,  in 
my  opinion,  can  be  employed  in  the  manufacture  of 
hydraulic  gypsum  only  in  cases,  in  which  the  gypsum 
does  not  come  in  immediate  contact  with  the  fuel,  as, 
for  instance,  in  laboratory  experiments,  in  which  the 
burning  is  done  with  gas.  Where  coal  is  used,  the 
ashes  of  it  as  well  as  the  reducing  carbon  are  bound 
to  contaminate  and  spoil  the  calcined  product.  More- 
over, temperatures  as  high  as  these  are  almost  out 
of  the  question  in  practical  operation. 


47 


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Cleveland,  O. 


Send  for  Book  " 


THE  OLD  METHOD 


ORENSTEIN-ARTHUR   KOPPEL  CO, 

PITTSBURGH 

New  York         Chicago         Baltimore         Boston         San  Francisco        Denver 


THE  NEW  METHOD 


BELT  CONVEYORS 

For  Use  in 

Cement  Plants 

Have  No  Equal 

We  have  some  interesting 
data  on  this  subject  which 
is  yours  for  the  asking. 

Robins  (Jonveying  [jell  [o, 

THOMAS  ROBINS        PflOQaip    U      I  C.  KEMBLE  BALDWIN 
President  I  UOuQIU,  Mi  Ji       Chief  Engineer 

NEW  YORK  CHICAGO 


WELLER- 


Shows 

Crushed  Stone 
Elevator. 

Send  for  Catalog. 


We  manufacture  these  in  all 
designs  and  for  handling  all 
classes  of  materials.  Furnished 
with  wood  or  iron  frames,  and 
of  any  desired  capacity. 
We  furnish  machines  complete,   ready   for 
work,  or  the  iron  work  alone. 

Ask  about  our  Belt  Conveying  and  Power 
Transmitting  Machinery. 


Weller  Mfg.  Co. 

Chicago. 


Link-Belt  Company 

Philadelphia  Chicago  Indianapolis 


Manufacturers  of  Conveying  and  Power 
Transmission  Machinery,  Sprocket  Wheels 
Etc.  for  Gypsum  Plants  and  Cement  Mills. 


Our   FLINT-RIM   Sprocket 
Wheels  are   Durable 

and  are  made  to  fit  the  original  "EWART"  and  other 
chains  of  our  manufacture  of  standard  pitch. 


Improved  Belt  Conveyor 
Idler 

Made   of    Pressed    Steel,    with    closed    ends---fixed   to 
through  shafts— which  turn  in  dust-proof  bearings. 


PECK  CARRIER 

The  Standard  Machine  for  handling  Cement,  Crushed 
Rock,  Hot  and  Cold  Clinker,  Coal,  Etc. 

Write  for   Booklet  and  Prices. 


fi'AJFEWIREASONS  FOR  ESTABLISHING  A 

Portland  Cement  Plant 
OR    Gypsum  Mill 


On  the  Rails  of  the 


SantaFe 

^  W 


RAW  MATERIALS— High  grade,  ex- 

tensive  deposits,  cheaply  handled. 
CHEAP  FUEL— Natural  gas,  coal,  oil. 

LARGE  MARKETS— The  western  half 
of  the  United  States,  an  immense  con- 
sumption, increasing  each  year. 

LABOR — At  reasonable  wages,  free  from 
disturbances. 

TRANSPORTATION  FACILITIES— 

The  best. 

Correspondence  from  interested  parties  solicited 


WESLEY  MERRITT 

Industrial  Commissioner  Santa  Fe  System 

RAILWAY  EXCHANGE,  CHICAGO 


RETURN     CIRCULATION  DEPARTMENT 

202  Main  Library 


LOAN  PERIOD  1 
HOME  USE 

2 

3 

4 

5 

6 

ALL  BOOKS  MAY  BE  RECALLED  AFTER  7  DAYS 

Renewals  and  Recharges  may  be  made  4  days  prior  to  the  due  date. 

Books  may  be  Renewed  by  colling     642-3405. 

DUE  AS  STAMPED  BELOW 


1937 


JAN  04  IS 


CIRCULATION   fepf 


FORM  NO.  DD6, 


UNIVERSITY  OF  CALIFORNIA,  BERKELEY 
BERKELEY,  CA  94720 


@$ 


THE  MANUFACTURE 


........................... 


HYDRAUL1 


AND  OTHER 


GYPSUM   PRODUCTS 


M.    GLASENAPP 


TRANSLATED  BY  DR,  W.  MICHAELIS,  JR. 


PIIIOB,  5O  CENTS 


ubliah«d   by 

CEMENT  &  ENGINEERING   NEWS 
GHICAJGO,  ILL. 


THIS  BOOK  IS  DUE  ON  THE  LAST  DATE 

^  2-  - 


AN  INITIAL  FINE  OF  25  CENTS 


«o  FAILU*E  TO   RETURN 

BOOK   ON    THE   DATE   DUE.    THE   PENALTY 

CENTS  ON 


OERDUE. 


°AY 


APR     4  1948 

__  * 

/ 


CIRCULATION  DEPT. 


LD  21-100m-8,'34 


YC   13894 


GENERAL  LIBRARY  -  U.C.  BERKELEY 


